Conventionally, an assembly line includes workstations coupled to each other to execute various operations on workpieces. Typically, different workstations require different lengths of time to execute their respective operations on the workpieces. As a result, performance of one workstation can affect that of another workstation in the assembly line. For example, a workstation that requires a longer time to execute its operations on a workpiece will slow down preceding or subsequent workstations. This typically causes periods of inactivity particularly for a subsequent workstation which will be idle while waiting to receive a workpiece from the workstation. In addition to the above, output of workpieces of the assembly line is adversely affected when just one workstation is inoperable.
Assembly line balancing to minimize the periods of inactivity of workstations increases output of workpieces. Such line balancing includes scheduling workpieces for workstations as described in, for example, U.S. Pat. No. 5,229,948 which discloses a method for optimizing a serial manufacturing system based on characteristics and status of workstations. In the method of U.S. Pat. No. 5,229,948, buffers are used for temporary storage of workpieces for workstations. However, buffers are not always appropriate, as for example, in an assembly line for component placement of circuit boards. This is because buffers increase a time period between component placement and reflowing. As a result, solder paste applied to the boards may be too dry if the time period is too long. Furthermore, a large number of workpieces in a buffer for a workstation is typically reduced by slowing down or even temporarily stopping a preceding workstation in executing operations.